1. Field of the Invention
This invention is related to the field of echo cancellation in communication networks employing hybrid transformers, and more particularly to an echo cancellation system that uses a mixed mode Least Mean Square (LMS) adaptive balance filter system which improves echo cancellation performance.
2. Description of the Relevant Art
Telephone networks commonly employ two-wire bi-directional communication lines which connect a subscriber, such as a telephone set in a home or office, to the network central office. In general, two-wire bi-directional communication lines are used in the subscriber loop for reduced cost. However, other communication paths in the telephone network, such as trunks between central offices, use four-wire communication lines. Four-wire communication lines have separate transmit and receive paths and provide better transmission quality due to the fact that each path may be separately optimized. Another advantage of four-wire lines is that communication techniques such as frequency division multiplexing (FDM), which require separate transmit and receive paths, may be employed. Hence, a means is necessary to convert two-wire bi-directional signals to separate transmit and receive signals on a four-wire communication line.
Central office line controller circuits frequently use Hybrid transformers to provide two wire to four wire circuit conversion. Hybrid transformers are also widely used in digital terminal equipment such as digital cordless telephone base stations, digital telephone answering devices, etc.
With the use of a hybrid transformer, four-wire communication lines may be used in most of the telephone network, providing increased transmission quality while less expensive two-wire lines may be used in the subscriber loop. However, the problem of impedance mismatch is introduced at the two wire to four wire conversion point, namely at the hybrid. This impedance mismatch results in the transmit path signal reflecting back to the receive path at the conversion point, which is commonly referred to as "echo". The amount of echo a hybrid generates depends on the quality of the hybrid to separate the receive and transmit paths and the line load condition. On a heavily loaded long loop, the echo effect will be larger. Typical echo is about 10 to 16 dB below the source signal without an echo canceller. This results in a very annoying effect if the echo is strong and has a long delay, such as 30 milliseconds or more. Possibly worse, in some cases, the echo can cause telephony equipment to malfunction. This is a common problem on full-duplex modems, DTMF detectors on telephone answering machines, and other such devices. Hence, echo cancellation is commonly used in such circuits to reduce echo problems.
Another situation which makes echo cancellation difficult is what is commonly referred to as "double-talker". Double-talker occurs where the communication line is transmitting and receiving simultaneously. By way of example, consider a digital telephone answering machine with a dual-tone multifrequency (DTMF) detector. An answering machine typically includes a DTMF detector to enable a person to control his answering machine locally or from a remote telephone keypad using DTMF tones, i.e. touch 2 for forward, 3 for rewind, 4 for stop, and 5 to leave a memo, etc. When the answering machine is in play back mode, the played speech on the transmit path echoes back to the receive path. If the user at a remote telephone is also entering DTMF commands, this may cause the DTMF detector to not be able to detect the remote DTMF commands. In other words, if speech is being played, such as the answering machine greeting or a recorded message, and is echoed back to the answer machine, and if the DTMF detector is receiving tones, the DTMF detector may have difficulty detecting the DTMF signals which are mixed with the presence of speech. As a result, the answering machine may miss the command. Hence, employment of an echo canceller in this case would be useful.
Another situation where echo can be harmful is when a subscriber device, such as an answering machine, generates a DTMF signal, and the signal reflects back at the hybrid and causes a false detection by the DTMF detector. It is readily apparent that an echo canceller will help in this situation.
Prior art echo cancellation methods include measuring the impulse response of the hybrid and then constructing a balancing filter that has the same characteristics as the hybrid. The filter receives the transmit path signal and generates an estimate of the echo. On the receive path, the echo estimate is subtracted from the actual received signal. The result is the echo canceled transmit signal.
If the line or transmission condition never changes and the response of the hybrid can be easily and accurately measured, a balancing filter echo canceller with fixed characteristics works sufficiently. Unfortunately, in a real-world telephone network, the line and transmission conditions constantly change. With these changes, it is essential to adaptively change the characteristics of the filter, namely the filter coefficients, quickly so that the echo cancellation is timely and accurate. An adaptive balance filter updates its coefficients based on the transmit path signal and the error signal. The error signal is the difference between the actual received signal and the echo estimate.
Often echo canceling balance filters are implemented as digital signal processors which adaptively compute the coefficients of the filter. One of the popular methods to compute filter coefficients are the least mean square (LMS)methods because of their low computational complexity and good stability if designed properly. LMS algorithms can be further classified as full LMS, sign-sign, sign-error, leaky LMS, normalized LMS, etc. Each version has its advantages and disadvantages.
A few criteria are commonly used in evaluating the quality of an echo canceller. The first is the speed of convergence, including the speed of initial convergence and the speed to regain convergence when the line condition changes. This is especially important in the case of an echo canceller designed as part of a DTMF detector because when a connection is established on the line, a DTMF signal can be immediately generated. Also, a valid DTMF signal can be as short as 40 milliseconds or less, depending upon the telephony standards of the particular country of interest.
In addition to the convergence time, another criteria used to evaluate an echo canceller is accuracy, which is the amount of echo cancellation. The following equation measures the echo cancellation: ##EQU1## In the above equation, x(n) is the transmit path signal, e(n) is the error signal, n is a sample index and N is the measurement frame size. The larger the cancellation result, the better the echo cancellation which the echo canceller achieves. In some telecommunications applications, a minimum cancellation result is required. For example, the DECT standard requires at least 24 dB cancellation at the near-end. Clearly, the goal is to design an echo canceller that converges fast, and cancels echo well.
A third echo canceller criteria is the ability to handle the double talker situation described previously. In this case, the received signal is the sum of the echo plus the far-end signal. The goal is to cancel out the echo as much as possible while minimizing the distortion to the far-end signal.
Finally, a fourth criteria is the complexity of the method used to calculate the filter coefficients. A simple but effective solution saves DSP code space and processing power, and is very much related to the cost of the product.
Hence, it is readily observed that an improved system and method is desired which performs echo cancellation in a communications channel in which a hybrid transformer performs two-wire to four-wire conversion.